Hi,
Don't worry, it is just a PHP warning.
If your site is in production, you should deactivate the PHP warning message and log them in a log file.
(You can change it in the Joomla global configuration).
Anyway, the problem is in maximenu, not in HikaShop.
Please contact them to fix this warning.
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.